1. Field of the Invention
The present invention relates to improvements of a magnetic shield for shielding intense magnetic fields, more particularly, improvements of a magnetic shield having a large area to prevent magnetic leakage in wide ranges.
2. Prior Art
To avoid adverse effects of magnetic fields generated from magnets or other substances, the art of magnetic shields which are made by using plate-shaped or sheet-shaped superconductors refrigerated below a critical temperature where superconductivity is generated is known as a conventional art for magnetically shielding a certain internal space. To achieve the object of shielding such magnetic fields, the second class superconductor which operates in the mixture region of the superconducting and normal conducting conditions is used more preferably than the first class superconductor, since the upper critical temperature of the second class superconductor is higher than that of the first class superconductor.
The maximum magnetic field shielding intensity of a superconductor, that is, the shielding intensity of a superconductor having a shape of a plate, sheet, film or membrane (in the general explanations described below simply referred to as "layer") for completely shielding external magnetic fields is significantly dependent on the class, size and shape of the superconductor. As pointed out in the patent specifications cited below by the inventors of the present invention, the maximum magnetic field shielding intensity of a superconductor increases abruptly as its thickness increases in a limited range. If the thickness increases over a certain value, the increasing rate of the maximum magnetic field shielding intensity becomes gentle. This indicates that an inflexion point is present on a curve which represents the relationship between the thickness of the superconductor and its maximum magnetic field shielding intensity. Considering this phenomenon, the inventors of the present invention have proposed a magnetic shield, the maximum magnetic field shielding intensity of which is significantly enhanced by using superconductor layers, the thickness of which is made smaller than that corresponding to the reflection point of the magnetic field shielding intensity, by laminating a superconductor layer with a normal conductor layer such as aluminum foil and by increasing the number of laminated layers (Japanese Laid-open Patent Appln. 61-183979, U.S. Pat. No. 4,803,452, U.S. Pat. No. 4,797,646, Can. PAT. 1261050, EP Appln. 86 101613. 7-2208).
To expand the range of a magnetic shielding space by enlarging the area of a magnetic shield, the size of a single superconductor layer is limited owing to the limitation in the production requirements of superconductors. In the case of producing larger superconductor layers exceeding the limitation, an art for enlarging such a magnetic shield by mutually overlapping the end sections of a plurality of oxide superconductor ceramic plates for example and by sticking the end sections together with conductive adhesive is known (Japanese Laid-open Patent Appln. 63-313897). The inventors of the present invention have also proposed a magnetic shield made by sticking a plurality of small superconductor pieces onto the external or internal surface of a cylinder with its one end closed (Japanese Laid-open Patent Appln. 1-302799).
These days, superconductor magnets are made larger to generate more intense magnetic fields. Because of this enlargement, the space ranges affected by the intense magnetic fields are also expanded. To shield unnecessary permeation of intense magnetic fields, magnetic shields which can securely shield intense magnetic fields over large areas have been requested. Such magnetic shields are applied to superconductor motors and superconductor generators, as well as superconductor magnets themselves used in linear motor cars and electromagnetic propellent ships.
The maximum magnetic field shielding intensity of a plate-shaped magnetic shield for completely shielding a magnetic field is apt to become lower at the external peripheral section than at the central section of the magnetic shield. In the case of a disc-shaped magnetic shield for example, the maximum magnetic field shielding intensity is lower at its peripheral section farther away from its central section in the radial direction. For this reason, although complete shielding is possible at the central section, a part of magnetism permeates the external peripheral section, causing magnetic leakage. Consequently, to completely shield a constant external magnetic field permeating a surface area, shielding is necessary at the central section of the surface area of the magnetic shielding plate by using a magnetic shield having the maximum magnetic shielding amount exceeding the external magnetic field intensity generated at the end of the surface area. As a result, the size of the magnetic shield must be considerably larger than the surface area.
A serious problem in the magnetic shielding process is the generation of a flux jump phenomenon wherein magnetic flux permeating the external peripheral section of the magnetic shield flows abruptly to the central section of the magnetic shield. If this occurs, the magnetic shield is heated locally and its superconducting condition is converted into a normal conducting condition, thereby causing magnetic field leakage over the entire magnetic shield. If this flux jump occurs once, the magnetic shield cannot act as a superconductor and completely loses its magnetic shielding function. Since the amount of generated heat is greater as the transfer distance of magnetic flux is larger, it is difficult to stably maintain the superconducting condition in an intense magnetic field.
In the magnetic shield of the above-mentioned prior art, which is made large by mutually laminating and sticking a plurality of superconductor ceramic plates, superconductive shielding current flows across every two ceramic plates. The structure of the magnetic shield is thus considered to be the same as that comprising a single superconductor, thereby being apt to cause the danger of losing the magnetic shielding effect due to the generation of the flux jump.
In the case of a tubular magnetic shield formed by pressing and sticking superconductor pieces (comprising superconductive low-melting-point alloy powder) onto the surface of a tube, the structure of this magnetic shield is the same as that of a tube formed to have a large surface area made by using a single superconductor, since the adjacent superconductor pieces are joined by the superconductor alloy. Accordingly, the maximum magnetic shielding amount of the magnetic shield is lower at the upper peripheral section of the opening area of the tube and lowest at the sealed end section of the bottom of the tube, causing the danger of losing the magnetic shielding effect due to the generation of the flux jump at such sections.
As a countermeasure for reducing the effect of the flux jump, a plurality of small through holes are provided in the surface area of a thin superconductor film such that the transfer range of the magnetic flux is limited to reduce heating and prevent a chain reaction of magnetic flux flow (Japanese Laid-open Patent Appln. 63-233577, U.S. Pat. No. 4,828,931, Can. Pat. 1296089, UK Pat. 2203909, German Pat. Appln. P38-09452. 5-34, FR 8803822). Even when a magnetic shield is enlarged, however, it still has a problem of reduction in the maximum magnetic field shielding intensity at its peripheral section.